1. Field of the Invention
The present invention relates generally to thermoelectric devices. Particularly, the present invention relates to thermoelectric devices and a method of fabricating the same.
2. Description of the Prior Art
Many types of industrial equipment require cooling or heating during their operation. Typical examples include semiconductor process equipment, pharmaceutical and biotechnology fermentation/separation vats, machine tools, air conditioners, plastic molding/extrusion equipment, analytical equipment, welding equipment, lasers, etc. One common way to provide the required cooling or heating is with a re-circulating coolant temperature control unit or chiller. A typical chiller consists of a Freon-based refrigeration loop connected to a recirculating coolant loop via a heat exchanger. However, as the world community becomes increasingly concerned about ozone depletion and global warming, a replacement for the standard Freon-based refrigeration technology is needed. Thermoelectric technology offers a clean, environmentally friendly, solid state alternative.
Thermoelectric cooling was first discovered by Jean-Charles-Athanase Peltier in 1834, when he observed that a current flowing through a junction between two dissimilar conductors induced heating or cooling at the junction, depending on the direction of current flow. This is called the Peltier effect. Practical use of thermoelectrics did not occur until the early 1960s with the development of semiconductor thermocouple materials, which were found to produce the strongest thermoelectric effect. Most thermoelectric materials today comprise a crystalline alloy of bismuth, tellurium, selenium, and antimony.
Thermoelectric devices are solid-state devices that serve as heat pumps. They follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. The principal difference is that thermoelectric devices function with solid state electrical components as compared to more traditional mechanical/fluid heating and cooling components.
The circuit for a simple thermoelectric device generally includes two dissimilar materials such as N-type and P-type thermoelectric semiconductor elements. The thermoelectric elements are typically arranged in an alternating N-type element and P-type element configuration. In many thermoelectric devices, semiconductor materials with dissimilar characteristics are connected electrically in series and thermally in parallel. The Peltier effect occurs when the voltage is applied to the N-type elements and the P-type elements resulting in current flow through the serial electrical connection and heat transfer across the N-type and P-type elements in the parallel thermal connection.
Typical construction of a thermoelectric module consists of electrically connecting a matrix of thermoelectric elements (dice) between a pair of electrically insulating substrates. The operation of the device creates both a hot-side substrate and a cool-side substrate. The module is typically placed between a load and a sink such as liquid plates, surface plates, or convection heat sinks. The most common type of thermoelectric element is composed of a bismuth-tellurium (Bi2Te3) alloy. The most common type of substrate is alumina (96%). These typically range in thickness from about 0.010 inches (0.25 mm) to about 0.040 inches (1.0 mm). A description of conventional thermoelectric modules and technology is also provided in the CRC Handbook of Thermoelectrics and Thermoelectric Refrigeration by H. J. Goldsmid.
A typical thermoelectric device requires DC power in order to produce a net current flow through the thermoelectric elements in one direction. The direction of the current flow determines the direction of heat transfer across the thermoelectric elements. The direction of net, non-zero current flow through the thermoelectric elements determines the function of the thermoelectric device as either a cooler or heater.
U.S. Pat. No. 6,222,243 (2001, Kishi et al.) discloses a thermoelectric device comprising a pair of substrates each having a surface, P-type and N-type thermoelectric material chips interposed between the pair of substrates, electrodes disposed on the surface of each substrate and connecting adjacent P-type and N-type thermoelectric material chips to each other, and support elements disposed over the surface of each of the substrates for supporting and aligning the thermoelectric material chips on the respective electrodes between the pair of substrates. Each of the thermoelectric material chips has a first distal end connected to one of the electrodes of one of the substrates and a second distal end connected to one of the electrodes of the other of the substrates. The adjacent P-type and N-type thermoelectric material chips connected by the electrodes are interposed between the pair of substrates such that a line connecting centers of the adjacent P-type and N-type thermoelectric material chips is coincident with a diagonal of each of the adjacent P-type and N-type thermoelectric material chips. The substrate used in the Kishi et al. device is a silicon wafer. A disadvantage of using silicon wafers as a substrate is the brittleness of the wafer and the thermal stresses that occur at the junction of the substrate and the thermoelectric material chips.
U.S. Pat. No. 5,362,983 (1994, Yamamura et al.) discloses a thermoelectric conversion module with series connection. The thermoelectric conversion module is constituted by either rows of thermoelectric semiconductor chips or columns of thermoelectric semiconductor chips of the same type. This arrangement improves assembling workability as well as preventing erroneous arrangement. The substrate used in the Yamamura et al. device is a ceramic substrate. A disadvantage of using a ceramic substrate is the brittleness of the ceramic and the thermal stresses that occur at the junction of the substrate and the thermoelectric semiconductor chips.
Other disadvantages of current thermoelectric module technology require that the substrates be thick enough to withstand cracking. The thicker the module, the heavier the thermoelectric module becomes. Also material costs for the thicker substrates are higher. In addition, the use of silicon or ceramic substrates limits the size and shape of thermoelectric modules. For instance, should spacing requirements determine that a normal 0.025 inch (0.64 mm) boundary between the edge of a pad and the edge of the module is too large, a special abrasive process must be used to reduce the boundary. Due to its brittleness, this may lead to chipping of the ceramic substrate.
Also, the rigidity of the ceramic substrate and the thermal cycling of a thermoelectric module where the heating side of the module is trying to expand while the cooling side of the module is trying to contract cause a xe2x80x9cPotato Chip Effect.xe2x80x9d This Effect puts stresses on the thermoelectric chips and results in the eventual failure at the junctures between different mediums. These stresses increase as the module size increases. Furthermore, current thermoelectric module technology limits the available applications where these devices can be used. For instance, current thermoelectric module technology is not practical in applications having irregular and non-flat surfaces.
Therefore, what is needed is a thermoelectric module that is thinner than currently available thermoelectric modules. What is also needed is a thermoelectric module that can be used in applications having irregular and non-flat surfaces. What is further needed is a thermoelectric module that can be shaped and sized to fit the application.
It is an object of the present invention to provide a thermoelectric module that is thinner than currently available thermoelectric modules. It is another object of the present invention to provide a thermoelectric module that can be used in applications having irregular and non-flat surfaces. It is still another object of the present invention to provide a thermoelectric module that can be formed to odd shapes and to provide a way to make much larger devices than heretofore possible. It is yet another object of the present invention to provide a thermoelectric module with thin, flexible substrates.
The present invention achieves these and other objectives by providing a thermoelectric module with a flexible thin film substrate on one or both sides of the module. The thin film substrate provides the electrical connection between the P-type and N-type thermoelectric elements and the electrical isolation from the heat source or heat sink. The thin film substrate also functions as a heat transfer medium. A material for use as a thin film material is one that preferably has relatively good heat transfer, a broad operating temperature range, relatively high dielectric strength, and relatively high resistance to thermal cycling fatigue. An example of an acceptable material for use as a thin film material is polyimide. Other thin film materials with similar properties may also be used. An example of such a material is an epoxy-based film.
The thin flexible film is laminated or otherwise bonded to copper or other electrically conductive material. An example of ways to bond or laminate copper to the thin, flexible film include sputtering a conductive material onto the surface or using adhesives to bond the copper material to the surface. The electrically conductive material forms the electrical junction between the semiconductor elements of the module. The thin film material also provides electrical isolation between the electrical connections and the heat source or heat sink.
An external layer of thermally conductive material to enhance heat transfer between the thermoelectric module and a heat source or heat sink is typical but not necessary. The external layer is also laminated to or otherwise bonded to the opposite surface of the thin film substrate. The external, thermally conductive material may cover the entire outside surface of the thin film substrate, or it may mirror the electrical connection pads forming the junction between the semiconductor elements of the thermoelectric module.